The present invention relates to a fermentation process for the production of vitamin B12.
Vitamin B12 is an important vitamin for humans and animals. It is used to treat pernicious anaemia and peripheral neuritis, and is used as a dietary supplement. Vitamin B12 is also an important animal feed supplement as growth enhancer.
The term vitamin B12 is used to describe compounds of the cobalt corrinoid family, in particular those of the cobalamin group. The most used compound of this group is cyanocobalamin and as such the term vitamin B12 is sometimes used to refer to cyanocobalamin. In this specification the term vitamin B12 should be attributed its broad meaning so as to include all the cobalt corrinoids of the cobalamin group, which include in particular cyanocobalamin, hydroxocobalamin, methylcobalamin and 5xe2x80x2desoxyadenosylcobalamin, characterised by a cyano, hydroxyl, methyl or 5xe2x80x2-desoxyadenosyl radical respectively. The methylcobalamin and 5xe2x80x2desoxyadenosylcobalamin compounds are known to be unstable to light in isolated form and are easily transformed to hydroxocobalamin in aqueous solution.
For this reason, almost all commercial vitamin B12 preparations consist of the stable cyanocobalamin, which as such is not the chemical form in which vitamin B12 can be found in nature. In this specification the term natural vitamin B12 is defined so as to comprise all chemical forms of vitamin B12 naturally occurring in nature, cyanocobalamin thus being excluded.
Vitamin B12 is produced industrially by microbial fermentation, using almost exclusively Pseudomonas denitrificans and Propionibacterium species (as reviewed by Spalla et al, 1989 xe2x80x9cMicrobial production of vitamin B12xe2x80x9d, In: Biotechnology of vitamins, pigments and growth factors, E. J. Vandamme ed., Elsevier, London, N.Y., pp. 257-284). Contrary to Pseudomonas, Propionibacteria are food-grade. Processes using Propionibacterium species thus have the advantage that they allow to formulate natural vitamin B12 together with the biomass in which it is produced, as recently described in EP-A-0 824 152. Such processes avoid the conversion of natural vitamin B12 into the cyanocobalamin form by chemical processes including cyanidisation followed by extraction and purification steps using organic solvents. The chemical conversion step and any subsequent purification steps cause this production process to be expensive, unsafe to the operators and environmentally unfriendly.
Propionibacteria are Gram-positive bacteria capable of producing valuable compounds in a variety of industrial processes. Propionibacteria are, for instance, important in the production of cheese, propionic acid, flavours and vitamin B12.
Propionibacteria are, as the name suggested, characteristic in the production of propionic acid. Glucose is commonly used as carbon source, but other substrates, i.e. fructose, mannose, galactose, glycerol and milk, can be used for growth. Besides propionic acid, acetic acid is produced under anaerobic conditions, with a ratio of 2:1 for propionic acid : acetic acid. The production of propionic acid is a clear advantage over other species as this compound is toxic in low levels for many other organisms, like lactic acid bacteria, acetic acid bacteria and yeasts. As a result there is little chance of contamination with other microorganisms during fermentation. The upper tolerance level for propionic acid for Propionibacterium is approximately 20-40 g/l (with a fermentation around pH 7): this is the level where the propionic acid starts to inhibit growth. The undissociated propionic acid is the actual toxic component for Propionibacteria, as is shown by Nanba et al. (1983, J. Ferment. Technol., 61: 551-556) for Propionibacterium shermanii. The specific growth rate decreases rapidly for undissociated propionic acid concentrations above 5 mM. This effect is also demonstrated by Blanc et al. (1987, Bioproc. Eng., 2: 175-179) for P. acidi-propionici, where the growth rate is drastically reduced above a pseudo critical value of 4 mM propionic acid. This implies that in fermentations with a pH around 7.0 propionic acid concentrations above 40 g/l are only reached with very low growth rates. This maximum amount of propionic acid produced in such fermentations results in a maximum of 25-35 g/l biomass that can be reached. Propionic acid concentration is thus one limiting factor for biomass growth and thereby for high vitamin B12 yield.
Several Propionibacterium species are capable to produce vitamin B12 in large scale fermentation processes. The process is described as a two-stage fermentation with a 72-88 hours anaerobic fermentation followed by a 72-88 hours aerobic phase. The vitamin B12 concentration in the cells rapidly increases in the aerobic phase, with typical values of 25-40 mg vitamin B12/l (see e.g. DE 1 239 694, U.S. Pat. No. 3,411,991, or in: Biochemical engineering and biotechnology handbook, 1991, B. Atkinson ed., Macmillan Publishers Ltd, pp: 1213-1220). Anaerobic growth followed by an aerobic phase with limited growth is important for economic production of vitamin B12 using Propionibacterium species. This requirement, however, limits the amount of biomass to 25-35 g/l as described above. Several attempts have been made to overcome the barrier of propionic acid toxicity in order to increase biomass and thereby the yield of vitamin B12.
Alternated anaerobic-aerobic phases are e.g. suggested to reduce the amount of acids (Ye et al., 1996, J. Ferment. Bioeng. 85: 484-491). In the aerobic phase the propionic acid is converted to less toxic acetic acid, with simultaneous formation of vitamin B12. The relative yield of vitamin B12 has been increased, but the final titre is rather low. This is probably due to inhibition early in the synthesis of vitamin B12 and/or other oxygen related products limiting the synthesis of vitamin B12. The final vitamin B12 produced with this method is 9 mg/l compared to 4.5 mg/l with the fully separated anaerobic and aerobic phases. Both values are rather low for vitamin B12 production with Propionibacteria.
The suggestion to use immobilized cells is mainly focused on the production of propionic acid (Rickert et al., 1998, Enzyme Microb. Technol. 22: 409-414). The propionic acid production is greatly enhanced. Use of this option for vitamin B12 production (which is not mentioned by Rickert et al.) will imply harvesting of the vitamin B12-containing cells with the immobilization material. This is only feasible when the additional cost for the immobilization equipment as well as the immobilization material itself are competitive with the current technology. Yongsmith et al. presented the production of vitamin B12 with immobilised cells of Propionibacterium sp. strain arl AKU1251. The maximum vitamin B12 concentrations is in the range of 14-16 mg/kg, which is no improvement of the production with freely suspended cells, as described before (Yongsmith et al. 1983, J. Ferment. Technol. 61: 593-598).
Although Propionibacteria can grow under aerobic conditions, the production of corrinoids (i.e. the general name for vitamin B12 and its precursors) is absent above a dissolved oxygen concentration of 0.19 mM=6 mg O2/L. The lower the oxygen concentration the higher the corrinoid production is with a maximum corrinoid production under non-aerated conditions (Quesada-Chanto et al., 1998, Appl. Microbiol. Biotechnol. 49: 732-736). Oxygen concentration is a limiting factor for vitamin B12 synthesis.
A repeated fed-batch fermentation with an anaerobic phase followed by an aerobic phase and withdrawal of broth at the end of the aerobic phase is not possible. According to Quesada-Chanto et al. (1998), production of the corrinoids is optimal under anaerobic conditions, whereas small amounts of oxygen reduce the production of corrinoids. These findings are supported by the results obtained in example 1. A repeated fed-batch process with an aerobic and anaerobic phase in one fermenter therefore is, not economically feasible.
GB patent 846,149 describes a continuous process for the synthesis of vitamin B12. This process comprises fermenting Propionibacterium in a nutrient medium under anaerobic conditions in a first zone while adding nutrients to this zone, passing cell-containing medium from the first zone into a second zone which is under microaerobic conditions and withdrawing cell-containing medium containing vitamin B12 from this second zone. The concentration of cells and the volume of medium in both zones is maintained substantially constant by continuous fill and draw operations. This process leads to the synthesis of up to 12 mg/l of vitamin B12, which is no improvement compared with a more classical method of production.
There is thus still a need for Propionibacterium-based fermentation processes for the production of vitamin B12 with further improvements in efficiency and/or vitamin B12 yield.
The present invention provides a process for producing vitamin B12 (and precursors thereof having detectable vitamin B12 activity) which is not a continuous process and which comprises the steps of:
(a) culturing a strain of the genus Propionibacterium in a first fermenter under anaerobic conditions to obtain a culture of Propionibacterium,
(b) transferring at least part of the culture obtained in (a) to a second fermenter and subjecting this culture to oxygen,
(c) replacing in the first fermenter part of the volume transferred in (b) with fresh culture medium, and
(d) repeating steps (a), (b), and (c) at least once.
In this specification the term vitamin B12 should be attributed its broad meaning so as to include all the cobalt corrinoids of the cobalamin group, which include in particular cyanocobalamin, hydroxocobalamin, methylcobalamin and 5xe2x80x2desoxyadenosylcobalamin, characterised by a cyano, hydroxyl, methyl or 5xe2x80x2-desoxyadenosyl radical respectively. The term vitamin B12 further comprises any vitamin B12 precursor having vitamin B12 activity as detectable in the turbidimetric bioassay based on the growth response of Lactobacillus leichmanii ATCC 7830 as described in detail in: the United States Pharmacopoeia, The National Formulary, 1995, pp. 1719-1721, United States Pharmacopoeial Convention, Inc., Rockville, Md.
One aspect of the process of the invention concerns the relation between the fraction of the anaerobic culture which is transferred in (b) and replenished with fresh medium in (c) and, on the one hand, the growth rate of the anaerobic culture (in (a)) and, on the other hand, the time interval between the subsequent withdrawals. Preferably, the withdrawal volume (which is transferred in (b) and replaced in (c)) relates to the total working volume of the first fermenter as a function of growth rate of the culture in the first fermenter and time interval between each draw according to the equation:       ln    ⁢          xe2x80x83        ⁢          (                        working          ⁢                      xe2x80x83                    ⁢          volume                                      working            ⁢                          xe2x80x83                        ⁢            volume                    -                      withdrawal            ⁢                          xe2x80x83                        ⁢            volume                              )        =      growthrate    *          (              time        ⁢                  xe2x80x83                ⁢        interval        ⁢                  xe2x80x83                ⁢        between        ⁢                  xe2x80x83                ⁢        each        ⁢                  xe2x80x83                ⁢        draw            )      
In this equation the growth rate is expressed in hxe2x88x921. Typically, the growth rate may vary over the applied time interval and an average value may be applied in the formula. The time interval between each draw is expressed in hours. Preferably, the growth rate during the anaerobic phase is maintained in the range between 0.03 and 1 hxe2x88x921. The skilled person will appreciate that at constant growth rate, decreasing both the time interval and the withdrawal volume will approach a continuous process, which is not a preferred embodiment of the present invention.
One embodiment of the process of the present invention comprises the steps of:
(a) culturing a strain of the genus Propionibacterium in a first fermenter under anaerobic conditions,
(b) transferring at least 30 to 90%, preferably at least 40 to 90%, more preferably at least 50 to 90%,more preferably at least 60 to 90% and most preferably at least 70 to 90% of the culture volume obtained in (a) to a second fermenter and subjecting this culture to oxygen,
(c) replacing in the first fermenter the same volume as the one withdrawn in step (b) with fresh culture medium; and
(d) repeating steps (a), (b), and (c) at least once.
In another preferred embodiment of the process of the invention, the culture of a strain of the genus Propionibacterium under anaerobic conditions (step (a)) leads to at least 20 g/l dry biomass, preferably at least 30 g/l, preferably at least 40 g/l, preferably at least 50 g/l, preferably at least 60 g/l, preferably at least 70 g/l, preferably at least 80 g/l, more preferably at least 90 g/l and most preferably at least 100 g/l.
Knowing the withdrawal volume and the growth rate of the microorganism used, the skilled person can easily deduce from the formula the time interval between each draw and thus the duration of the whole process.
In another preferred embodiment of the process of the invention, the dissolved Oxygen concentration at inoculation of the anaerobic phase is less than 5% of air saturation, preferably less than 2.5%, and more preferably less than 1% of air saturation.
Preferably, the anaerobic conditions of the process of the invention are such that the oxygen uptake rate during the anaerobic phase (a) is no more than 2 mmol O2 lxe2x88x921hxe2x88x921, preferably no more than 1 mmol O2 lxe2x88x921hxe2x88x921, and most preferably approaches zero mmol O2 lxe2x88x921hxe2x88x921, as measured by mass-spectometry and gas flow analysis (see e.g. in: Basic Bioreactor Design, 1991, K. van""t Riet and J. Tramper, eds., Marcel Dekker Inc.).
In step (b) of the process of the invention, the culture is subjected to oxygen. Preferably, the oxygen uptake rate (OUR) during this aerobic phase of the process is at least 5 mmol O2 lxe2x88x921hxe2x88x921, more preferably at least 20 mmol O2 lxe2x88x921hxe2x88x921, still more preferably at least 40 mmol O2 lxe2x88x921hxe2x88x921, and most preferably at least 80 mmol O2 lxe2x88x921hxe2x88x921.
According to one embodiment of the invention there is provided a process wherein the aerobic second phase in (b) is performed in at least two serially connected aerobic fermenters. Most preferably the aerobic second phase in (b) is performed in plug flow mode, wherein e.g. the xe2x80x9csecondxe2x80x9d aerobic fermenter comprises a series of aerobic fermenters.
Suitable culture media for the production of vitamin B12 with Propionibacteria are well-known in the art (cf. Biochemical engineering and biotechnology handbook, 1991, B. Atkinson ed., Macmillan Publishers Ltd, pp: 1213-1220). In a preferred embodiment of the invention the culture medium is supplemented with 1-50 mM of one or more compounds selected from the group consisting of betaine, methionine and glutamine. Another preferred supplement for the culture medium is 5,6-dimethylbenzimidazole (DBI). DBI is preferably supplemented at 1-40 mg DBI per litre. Preferably, DBI is added to the culture medium at the start of the aerobic phase or during that phase.
In a preferred embodiment of the process of the invention the concentration of undissociated propionic acid is controlled such that it does not exceed 5 mM. This conveniently may be done by increasing the pH of the culture medium according to methods well known to the skilled person.
According to another embodiment of the invention, the temperature under anaerobic conditions is different from the temperature under aerobic conditions. Preferably, the temperature under anaerobic condition is higher than the temperature under aerobic conditions. More preferably, the temperature under anaerobic conditions is at least 2 degrees higher than the temperature under aerobic conditions, more preferably at least 4 degrees higher, more preferably at least 6 degrees higher, more preferably at least 8 degrees higher, more preferably at least 10 degrees higher and most preferably at least 12 degrees higher. For instance, the temperature under anaerobic conditions may be 36xc2x0 C. and under aerobic conditions 24xc2x0 C., or these temperatures may be 36xc2x0 C. and 30xc2x0 C., respectively.
In the process of the invention, preferably a strain of a Propionibacterium species is used which is selected from the group consisting of the classical or Dairy Propionibacteria as described in Bergey""s manual of systematic bacteriology, 1986, J. B. Butler, Williams and Wilkins, p 1346-1353. This group comprises inter alia the species P. freundenreichii with subspecies freudenreichii and shermanii, P. thoenii, P. jensenii and P. acidipropionici. More preferably the strain P. freundenreichii CBS 929.97 is used. P. freundenreichii CBS 929.97 was deposited Jul. 10, 1997 at the Centraal Bureau voor Schimmelcultures, Baarn, The Netherlands.
In one embodiment of the invention, the process of the invention is performed using Propionibacteria strains that are genetically modified by means of classical mutagenesis and/or recombinant DNA technology. Classically mutagenised strains can be propionic acid-resistant strains of the genus Propionibacterium, such as P. shermanii NOC 11012 and P. freudenreichii NOC 11013, as disclosed in U.S. Pat. No. 4,544,633. A propionic acid-resistant strain of Propionibacterium is herein defined as a strain which shows approximately equal (i.e. less than 10% difference) growth rates when compared in identical media with and without 20 g propionic acid/l.
Transformed Propionibacteria strains that have been genetically modified by recombinant DNA technology are exemplified in J 08-056673.
One of the advantages of the process of the invention is the throughput increase of the fermentation. The anaerobic phase is considerably reduced. For a growth rate of 0.06 hxe2x88x921 the output of the anaerobic phase is three fermenter volumes in 72 hours, compared to one fermenter volume for the classical process. In addition, the invention reduces the turnaround time of the fermentation, with ten consecutive fills and draws, to 20% of the classical process.
Another advantage of the process of the invention is that the size of the fill and draw volumes may be relatively large. Consequently, the growth inhibiton caused by high concentration of propionic acid under anaerobic conditions does not occur, leading to an increase in the biomass and thereby an increase in vitamin B12 yield. The amount of vitamin B12 formed using the process of the invention may be 20 to as high as 200 mg/kg mesh.